Various salts of the acids of phosphoric acid, usually orthophosphoric acid or pyrophosphoric acid are commonly employed as the acid factor in combination with a carbonate factor in leavening systems. Because the calcium and sodium salts exhibit different reaction profiles, both find use as leavening acids in different applications. Both sodium and calcium salts have been widely used as the acid factor in leavening systems.
A baking preparation employing one form of calcium acid pyrophosphate is described in U.S. Pat. No. 990,699 to Brown et al. The calcium salt is obtained by roasting calcium orthophosphate thereby removing about 2 moles of water from each mole of the orthophosphate. Calcium acid pyrophosphate prepared in such manner has a desirable reaction profile for some applications indicating a rapid rate of reaction. The product is usually impure having large amounts of other phosphates including unreacted calcium monophosphate.
The use of sodium acid pyrophosphate as an acid factor in bakery leavening is known but an undesirable flavor has been observed. Mixing a calcium acid phosphate salt with sodium acid pyrophosphate has been found to reduce or eliminate the taste and also control the evolution of gas after mixing the baking ingredients. In U.S. Pat. No. 1,834,747 to Stokes et al. there is described baking powder formulas which contain the usual sodium bicarbonate together with alkaline earth metal phosphates such as mono-calcium phosphate in admixture with sodium acid pyrophosphate. It is reported that the mixture results in a slowing of the evolution of carbon dioxide as compared to sodium acid pyrophosphate alone thereby allowing a more desirable reaction profile. With variation in the amounts of the various salts it is reported that the evolution of gas during leavening can be controlled to provide varied reaction profiles depending upon the requirements.
The leavening properties of a certain calcium acid pyrophosphate was recognized in U.S. Pat. No. 1,847,799 wherein a completely dehydrated, neutral calcium pyrophosphate was employed as a filler in baking powder compositions. It was observed therein that calcium phosphate dried at only one hundred degrees losses its water only slowly while at higher temperatures all of the water is removed. Incorporation of neutral calcium pyrophosphate provided storage stability to baking powders such that the leavening action was not lost upon prolonged standing or exposure to atmospheric moisture.
In U.S. Pat. No. 2,133,286 there are disclosed several calcium acid tetraphosphates useful as acid factors in leavening with a carbonate factor such as sodium bicarbonate.
The reaction rate of monocalcium phosphate during leavening is modified by the addition to the system of calcium acid pyrophosphate according to U.S. Pat. No. 2,263,487. The pyrophosphate is prepared by heating monocalcium phosphate to a temperature in the range of from about 200.degree. C. to about 250.degree. C. whereby a coating of the pyrophosphate is formed around at least a portion of the exterior of the monocalcium phosphate particles. It was observed that the calcium acid pyrophosphate coating on the monocalcium phosphate particles lowered the rate of release of carbon dioxide when only 10 percent of the particle was converted to the acid pyrophosphate. Further, it was found that additional formation of calcium acid pyrophosphate on the monocalcium phosphate particles, up to total coating, did not serve to further lower the rate of release of carbon dioxide but rather prolonged the low rate. Gas evolution curves compared results obtained from calcium acid pyrophosphate alone with monocalcium phosphate having varying amounts converted to the pyrophosphate. The rate of reaction for calcium acid pyrophosphate alone indicated by percent of carbon dioxide release shows a fairly constant, slow rate providing only 40% of the available carbon dioxide after one hour.
The use of milk in the baking formula has been observed to cause a loss of elasticity and specific volume of the baked product (biscuits) when calcium acid pyrophosphate is employed as the leavening acid. Such effects are shown in U.S. Pat. No. 2,366,857 and were attributed to the reaction of calcium in the milk with calcium acid pyrophosphate. Such effects were found to be abated by the addition to the baking formula of small amounts of alkali metal hexa metaphosphate. The calcium acid pyrophosphate employed in this patent was prepared by the "Booth" method wherein phosphoric acid is reacted with calcium hydroxide in the form of quick lime with the acid at a temperature above 180.degree. C. but below 300.degree. C. The product of the reaction is a mixture of calcium salts including the pyrophosphate and monocalcium orthophosphate. According to this disclosure calcium acid pyrophosphate prepared by heating calcium orthophosphate may also be employed as the leavening acid improved by the addition of hexa metaphosphate.
More recently, in EPO 0 350 607 A2 there has been reported the use of acidic calcium pyrophosphate as a leavening agent in baking processes for which such compound was formerly believed to be unsuitable. More particularly, it has been observed to perform satisfactorily in ready-made doughs which are frozen for extended periods of time before baking. Formerly, acidic calcium pyrophosphate reacted quickly in the dough, making it useful only when the dough was immediately processed. The relatively slow action of the calcium acid pyrophosphate is reportedly achieved by providing it in highly pure form from the dehydration reaction of monocalcium phosphate or the monohydrate thereof in a thin layer on the walls of a closed reactor which is closely controlled so as to heat the thin layer to a temperature in the range of from about 255.degree.-330.degree. C. The identity of the product as calcium acid pyrophosphate was reportedly confirmed by x-ray powder diffraction. By carefully controlling the dehydration process it is claimed that calcium acid pyrophosphate is obtained having a purity of at least 95%. The product is found to be in admixture with small amounts of monocalcium phosphate and calcium metaphosphates.
From the above noted prior art there is a pattern of activity in the baking industry which has characterized the leavening action of calcium acid pyrophosphate as slow acting. In instances of reported fast reaction it would seem to depend upon the purity of the pyrophosphate, as the usual method for preparing the pyrophosphate disclosed in the literature provides a mixture of both monocalcium orthophosphate and calcium acid pyrophosphate.
The characteristics of a leavening system can be determined by means of a test commonly known in the industry as the "Dough Rate of Reaction" (DRR) test. Because the DRR test has been employed in research laboratories for a long period of time, the test has been found to be a reliable indicator of how a leavening system will operate in actual use. The test is described by Parks et al. in Cereal Chemistry, Vol. 37, pp. 503-518 July, 1960 hereby incorporated by reference. According to said test, the reaction characteristics of a leavening system, comprising an acid factor and a carbonate factor in a specified biscuit dough is determined. The characteristics are determined by measuring the amount of carbon dioxide released beginning with initial mixing of the dough ingredients and during subsequent time periods. The test is repeated at several different temperatures which are held constant throughout the test. By measuring the amount of carbon dioxide gas produced during specified time periods, a reaction profile (amount of CO.sub.2 /time) of the leavening system can be determined. The results of such tests are best viewed by means of graphical display of the data.
Previously, there has not been observed any calcium acid pyrophosphate, no matter how prepared, which exhibited a variable rate of reaction, i.e., remaining relatively slow acting for a time and then, without change of condition such as temperature, providing a more rapid rate of reaction and releasing a large amount of carbon dioxide relative to the amount theoretically available.